A detailed investigation of photocatalytic degradation of Acid Yellow 36 (AY36) has been carried out in aqueous heterogeneous medium containing ZnO as photocatalyst in a batch reactor. The effects of some parameters such as pH, catalyst loading, and ethanol concentration were examined. Solutions with initial concentration of 50？mg L？1 of dye, within the range of typical concentration in textile wastewaters, were treated at natural pH of 6.93 and catalyst concentration of 1？g L？1 after 180？min irradiation. Investigations on the active species indicated that hydroxyl radicals play the major role in the process. Experiments showed that the most efficient pH on the removal of the dye with photocatalytic degradation process was 8; however, acidic pH was favored for the dark surface adsorption. Electrical energy consumption per order of magnitude for photocatalytic degradation of AY36 has been also determined. 1. Introduction Textile processing industries nowadays are widespread sectors in developing countries. Among the various processes in the textile industry, dyeing process uses large volume of water for dyeing, fixing, and washing processes. The textile industry uses about 10000 different dyes and the worldwide annual production of dyestuffs is over 7 105 tons [1]. Among several classes of textile dyestuffs, the reactive dyes contribute about 50% [2] of the total market share and the most common group used as chromophore is the azo (70%), followed by anthraquinone [3]. It has been documented that dye loss in wastewaters could vary up to 50% during manufacturing or processing operations [2]. The release of those colored wastewaters in the ecosystem constitutes a dramatic source of pollution which causes these effluents to be toxic and mostly nonbiodegradable. Namely, the coloration is visible even in a very low dye concentration (below 1？mg？L？1) [4, 5]. Consequently, technological systems for the removal of organic pollutants such as adsorption on active carbon, reverse osmosis, ion exchange on synthetic adsorbent resins, ozonation, and biological methods were examined [6]. All of them have advantages and drawbacks but these processes have very high operating costs. However, these conventional treatment methods cannot effectively remove the dyes from effluents, so finding an effective technique is necessary [6]. Recently, an increasing attention has been devoted to developing dye treatment methods from waste stream at its source (ISO 14001, October 1996). An alternative to conventional methods, “advanced oxidation processes” (AOPs) have been developed
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